Network Layer

Mike FreedmanCOS 461: Computer Networks

http://www.cs.princeton.edu/courses/archive/spr14/cos461/

IP Protocol Stack: Key Abstractions

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Best-effort local packet delivery

Best-effort global packet delivery

Reliable streams

Applications

Messages

Link

Network

Transport

Application

Best-Effort Global Packet Delivery

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Circuit Switching (e.g., Phone Network)• Source establishes connection

– Reserve resources along hops in the path

• Source sends data– Transmit data over the established connection

• Source tears down connection– Free the resources for future connections

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Circuit Switching: Static Allocation

Q: Frequency-Division vs. Time-Division

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timetime

Packet Switching• Message divided into packets

– Header identifies the destination address

• Packets travel separately through the network– Forwarding based on the destination address– Packets may be buffered temporarily

• Destination reconstructs the message

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Packet Switching: Statistical (Time Division) Multiplexing

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Packets

• Intuition: Traffic by computer end-points is bursty!– Versus: Telephone traffic not bursty (e.g., constant 56 kbps)– One can use network while others idle

• Packet queuing in network: tradeoff space for time– Handle short periods when outgoing link demand > link speed

Is Best Effort Good Enough?

• Packet loss and delay– Sender can resend

• Packet corruption– Receiver can detect,

and sender can resend

• Out-of-order delivery– Receiver can put the

data back in order

• Packets follow different paths– Doesn’t matter

• Network failure– Drop the packet

• Network congestion– Drop the packet

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Packet vs. Circuit Switching?

• Predictable performance• Network never blocks senders• Reliable, in-order delivery• Low delay to send data• Simple forwarding• No overhead for packet headers• High utilization under most workloads• No per-connection network state

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CircuitPacketCircuitPacketCircuitCircuitPacketPacket

Network Addresses

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IP Address (IPv4)• A unique 32-bit number• Identifies an interface (on a host, on a router, …)• Represented in dotted-quad notation

00001100 00100010 10011110 00000101

12 34 158 5

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Grouping Related Hosts• The Internet is an “inter-network”

– Used to connect networks together, not hosts– Need to address a network (i.e., group of hosts)

LAN = Local Area Network

WAN = Wide Area Network

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host host host

LAN 1

... host host host

LAN 2

...

router router routerWAN WAN

Scalability Challenge• Suppose hosts had arbitrary addresses

– Then every router would need a lot of information– …to know how to direct packets toward every host

host host host

LAN 1

... host host host

LAN 2

...

router router routerWAN WAN

1.2.3.4 5.6.7.8 2.4.6.8 1.2.3.5 5.6.7.9 2.4.6.9

1.2.3.4

1.2.3.5

forwarding table 15

Hierarchical Addressing: IP Prefixes• Network and host portions (left and right) • 12.34.158.0/24 is a 24-bit prefix with 28 addresses

00001100 00100010 10011110 00000101

Network (24 bits) Host (8 bits)

12 34 158 5

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Scalability Improved• Number related hosts from a common subnet

– 1.2.3.0/24 on the left LAN– 5.6.7.0/24 on the right LAN

host host host

LAN 1

... host host host...

router router routerWAN WAN

1.2.3.4 1.2.3.7 1.2.3.156 5.6.7.8 5.6.7.9 5.6.7.212

1.2.3.0/24

5.6.7.0/24

forwarding table 19

LAN 2

Easy to Add New Hosts• No need to update the routers

– E.g., adding a new host 5.6.7.213 on the right– Doesn’t require adding a new forwarding-table

entry

host host host

LAN 1

... host host

LAN 2

...

router router routerWAN WAN

1.2.3.4 1.2.3.7 1.2.3.156 5.6.7.8 5.6.7.9 5.6.7.212

host

5.6.7.213

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1.2.3.0/24

5.6.7.0/24

forwarding table

host

History of IP Address Allocation

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Classful Addressing• In the olden days, only fixed allocation sizes

– Class A: 0*• Very large /8 blocks (e.g., MIT has 18.0.0.0/8)

– Class B: 10*• Large /16 blocks (e.g,. Princeton has 128.112.0.0/16)

– Class C: 110*• Small /24 blocks (e.g., AT&T Labs has 192.20.225.0/24)

– Class D: 1110* for multicast groups– Class E: 11110* reserved for future use

• This is why folks use dotted-quad notation!

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Classless Inter-Domain Routing (CIDR)

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IP Address : 12.4.0.0 IP Mask: 255.254.0.0

00001100 00000100 00000000 00000000

11111111 11111110 00000000 00000000

Address

Mask

for hosts Network Prefix

Written as 12.4.0.0/15

• Use two 32-bit numbers to represent network:Network number = IP address + Mask

Hierarchical Address Allocation• Hierarchy is key to scalability

– Address allocated in contiguous chunks (prefixes)– Today, the Internet has about 400,000 prefixes

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12.0.0.0/8

12.0.0.0/16

12.254.0.0/16

12.1.0.0/1612.2.0.0/1612.3.0.0/16

:::

12.3.0.0/2412.3.1.0/24

::

12.3.254.0/24

12.253.0.0/1912.253.32.0/19

12.253.160.0/19

:::

::

Obtaining a Block of Addresses• Internet Corporation for Assigned Names and

Numbers (ICANN)– Allocates large blocks to Regional Internet Registries

• Regional Internet Registries (RIRs)– E.g., ARIN (American Registry for Internet Numbers)– Allocates to ISPs and large institutions

• Internet Service Providers (ISPs)– Allocate address blocks to their customers– Who may, in turn, allocate to their customers…

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Long Term Growth (1989-2005)

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Today we are up to ~400,000 prefixes

Pre CIDR CIDR’s effect

DotCom Boom

Post Boom

Packet Forwarding

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Hop-by-Hop Packet Forwarding• Each router has a forwarding table

– Maps destination address to outgoing interface

• Upon receiving a packet– Inspect the destination address in the header– Index into the table– Determine the outgoing interface– Forward the packet out that interface

• Then, the next router in the path repeats

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IP Router

SwitchingFabric

Processor

Adapter

Adapter

Adapter

Adapter

Adapter

Adapter

data planecontrol plane

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Switch Fabric: From Input to OutputQueuePacket

BufferMemory

BufferMemory

QueuePacket

BufferMemory

BufferMemory

QueuePacket

BufferMemory

BufferMemory

Data Hdr

Data Hdr

Data Hdr

1

2

N

1

2

N

Separate Forwarding Entry Per Prefix• Prefix-based forwarding

– Map the destination address to matching prefix– Forward to the outgoing interface

host host host

LAN 1

... host host host

LAN

...

router router routerWAN WAN

1.2.3.4 1.2.3.7 1.2.3.156 5.6.7.8 5.6.7.9 5.6.7.212

1.2.3.0/24

5.6.7.0/24

forwarding table 35

CIDR Makes Packet Forwarding Harder• Forwarding table may have many matches

– E.g., entries for 201.10.0.0/21 and 201.10.6.0/23– The IP address 201.10.6.17 would match both!

201.10.0.0/21

201.10.0.0/22 201.10.4.0/24 201.10.5.0/24 201.10.6.0/23

Provider 1 Provider 2

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201.10.6.0/23

Longest Prefix Match Forwarding• Destination-based forwarding

– Packet has a destination address– Router identifies longest-matching prefix– Cute algorithmic problem: very fast lookups

4.0.0.0/84.83.128.0/17201.10.0.0/21201.10.6.0/23126.255.103.0/24

201.10.6.17destination

forwarding table

Serial0/0.1outgoing link

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Creating a Forwarding Table• Entries can be statically configured

– E.g., “map 12.34.158.0/24 to Serial0/0.1”

• But, this doesn’t adapt – To failures– To new equipment– To the need to balance load

• That is where the control plane comes in– Routing protocols

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Data Control Management

Time-scale

Packet (ns)Event

(10 ms to sec)

Human

(min to hours)

Tasks

Forwarding, buffering, filtering,

scheduling

Routing, signaling

Analysis, configuration

LocationLine-card hardware

Router software

Humans or scripts

Data, Control, & Management Planes

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SwitchingFabric

Processor

Q’s: MAC vs. IP Addressing

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• Hierarchically allocatedA) MAC B) IP C) Both D) Neither

• Organized topologicallyA) MAC B) IP C) Both D) Neither

• Forwarding via exact match on addressA) MAC B) IP C) Both D) Neither

• Automatically calculate forwarding by observing dataA) Ethernet switches B) IP routers C) Both D)

Neither • Per connection state in the network

A) MAC B) IP C) Both D) Neither• Per host state in the network

A) MAC B) IP C) Both D) Neither

IP Packet Format

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IP Packet Structure

4-bitVersion

4-bitHeaderLength

8-bitType of Service

(TOS)16-bit Total Length (Bytes)

16-bit Identification3-bitFlags 13-bit Fragment Offset

8-bit Time to Live (TTL) 8-bit Protocol 16-bit Header Checksum

32-bit Source IP Address

32-bit Destination IP Address

Options (if any)

Payload

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Conclusion• Best-effort global packet delivery

– Simple end-to-end abstraction– Enables higher-level abstractions on top– Doesn’t rely on much from the links below

• IP addressing and forwarding– Hierarchy for scalability and decentralized control– Allocation of IP prefixes– Longest prefix match forwarding

• Next time: transport layer44

Backup Slides

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IP Header: Version, Length, ToS• Version number (4 bits)

– Necessary to know what other fields to expect– Typically “4” (for IPv4), and sometimes “6” (for IPv6)

• Header length (4 bits)– Number of 32-bit words in the header– Typically “5” (for a 20-byte IPv4 header)– Can be more when “IP options” are used

• Type-of-Service (8 bits)– Allow different packets to be treated differently– Low delay for audio, high bandwidth for bulk transfer

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IP Header: Length, Fragments, TTL• Total length (16 bits)

– Number of bytes in the packet– Max size is 63,535 bytes (216 -1)– … though most links impose smaller limits

• Fragmentation information (32 bits)– Supports dividing a large IP packet into fragments– … in case a link cannot handle a large IP packet

• Time-To-Live (8 bits)– Used to identify packets stuck in forwarding loops– … and eventually discard them from the network

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IP Header: Transport Protocol• Protocol (8 bits)

– Identifies the higher-level protocol• E.g., “6” for the Transmission Control Protocol (TCP)• E.g., “17” for the User Datagram Protocol (UDP)

– Important for demultiplexing at receiving host• Indicates what kind of header to expect next

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IP header IP header

TCP header UDP header

protocol=6 protocol=17

IP Header: Header Checksum• Checksum (16 bits)

– Sum of all 16-bit words in the header– If header bits are corrupted, checksum won’t match– Receiving discards corrupted packets

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134+ 212

= 346

134+ 216

= 350

Mismatch!

IP Header: To and From Addresses• Destination IP address (32 bits)

– Unique identifier for the receiving host– Allows each node to make forwarding decisions

• Source IP address (32 bits)– Unique identifier for the sending host– Recipient can decide whether to accept packet– Enables recipient to send a reply back to source

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